Why can't Pokémon Ruby and Sapphire trade with Gold and Silver?

The break begins inside the Pokémon. Generation III rebuilt the data structure completely: every Pokémon gained a personality value drawn from a range of over four billion, which determines its newly introduced Ability and Nature as well as its gender, and the individual values beneath its stats were rescaled from 0 to 15 up to 0 to 31, with Hit Points and Special Defense gaining values of their own. A Generation II Pokémon, as a piece of data, has no slots for what a Generation III Pokémon is.

Trading compounded the problem. The record is plain that it is not possible to trade between Generation II and Generation III, full stop. Accounts of the era add the hardware half of the story, that the Advance's link cables and protocol differ from the Game Boy and Color's, and this Library flags the distinction honestly: the data overhaul is documented in Bulbapedia's Generation III record, while the link-protocol explanation circulates in era reporting without a primary Nintendo statement behind it in the sources opened for this entry.

The consequences shaped the generation. FireRed and LeafGreen exist partly because of the wall: the remakes were created as a result of Ruby and Sapphire's lack of backward compatibility, a way to bring the missing species forward into the new format. And for players, Ruby and Sapphire were a fresh start by force, a Pokédex of what Hoenn could catch, trade and receive from within its own generation only.

The break had a purpose players felt immediately: Ruby and Sapphire were a fresh start by force, a Pokédex of what Hoenn could catch, trade and receive from within its own generation only.